I agree about strings.

There already are a lot of real tracks that include piano. Besides, piano is probably the instrument most emulatable (?) through MIDI, mainly because the most common type of MIDI controller is a piano-type keyboard. If one is not satisfied with the sampled pianos in one's GM and other multi-instrument collections, there are very good piano sample libraries one could buy. (One I recently tried, inexpensive, and low on disk and other resources needed, yet a very nice sound, is called Pianissimo by Acoustica. It only costs around $70 (free demo version available for download), and combines sample-playing with virtual modeling, and only uses I think about 250MB disk space, as opposed to the large multi-GB sample libraries.

I disagree with Pat about strings though. Despite all the excellent string sample libraries around, I think it is hard to emulate string-playing well through MIDI. And, bowed strings are definitely under-represented in the current RealTracks, with I think perhaps one or two solo fiddles in bluegrass styles, if I remember correctly.

It would be nice to have a Real Strings package, of whole string sections playing chord progressions, etc.